Rain is a defining characteristic of the Pacific Northwest. Most of that region gets over 30 inches a year, and some areas get over 200. But human-caused climate change may alter this, according to the results of a recent study.

Increasing droughts in this normally rainy area spurred a team of scientists from the University of Pittsburgh to explore the region’s historical precipitation patterns. The scientists looked at lake bottom sediments in Washington state to determine how often droughts occurred, how long they lasted and how severe they were. The team then pieced together a 6,000-year timeline of precipitation and evaporation in the Pacific Northwest.

The results, published last week in the Proceedings of the National Academy of Science, show that El Niño events off the coast of Ecuador drive much of the precipitation fluctuations in areas such as Washington and Oregon. And while droughts have occurred throughout the 6,000 years the scientists analyzed, the length and severity of these dry periods have increased in the last century,.

The scientists suspect that rising ocean temperatures are driving these droughts—a stronger El Niño in the Pacific Ocean means less rain in the Pacific Northwest. As population and greenhouse gas production both increase, they warn that even this normally rainy area may end up short of water.